I just got off the phone with a representative of a Catholic publisher. He had some questions about the pronunciation of liturgical Latin.
His experience in digging into this issue is much the same as what I have found through the years, namely, people can get pretty worked up about this stuff.
Remember that language is… well.. language. There are no hard and fast rules to prescribe in human speech.
Sure, there are conventions and "standards" of pronunciation.
I think we have all heard that "BBC English" was the "standard" for broadcasting the the UK for a long time. The upper-midwestern accent of the US, such as Omaha, was the "standard" in American broadcasting to which everyone, such as Texans, had to conform. It is usually held that the Italian of Lucca in Tuscany is the "purest" Italian or the French of, if I remember properly, Orleans, is their reference point in France. In China, the official stand on Mandarin is the Beijing pronunciation.
Insofar as Holy Roman Church is concerned, since at least the time of Pius X the Roman pronunciation of Church Latin was considered the language standard. This has been reinforced more recently, in the time of Bl. John XXIII (e.g., in Ordinationes ad Constitutionem Apostolicam Veterum Sapientia Rite Exsequendam in AAS 54/6 (30 Maii 1962), p. 345 and n. 10.
So,… that would either make Rome the Omaha of the Church, or Omaha the Rome of American broadcasting…. hmmm….
For example, pick up your handy English edition of the Liber Usualis and you will find in the front a section called "Rules of Interpretation". You will find therein not only directions about how to sing a scandicus and climacus, but also the values of vowels, diphthongs and consonants.
The Liber says:
"In good Latin diction – listen to Roman professor lecturing in Latin – the tonic accent stands out clearly" etc.
"Our aim, in compliance with the wishes of his holiness Pius X, is to pronounce and speak Latin in the Roman Style so eminently suitable to Plainsong."
"Many have never learned the Roman pronunciation or know it imperfectly. Besides its great importance in Plainsong it makes for that uniformity which inspired the Vatican Edition itself: Unus Cultus, Unus Cantus. We therefore give a list of the correct pronunciation of the vowels and consonants to which reference can be made in case of doubt; it is advisable to peruse it from time to time."
The Liber is rather prescriptive, but… well… you have to say something more than "say it any way you want". Still, I need to observe that finding a Romano di Roma is getting harder and harder… especially a well-educated Romano di Roma who knows Latin. They exist, but they are getting rarer.
Also, note that the Liber is not talking about a French guy lecturing in Rome in Latin, but a Roman. All of us are going to tend, to a greater or lesser degree, depending on one’s "ear" to impose the sound inventory of the mother tongue even on the well-practiced Roman style of ecclesiastical Latin.
So, we mustn’t be overly rigid about these things and wind up breathing spittle-flecked imprecations against someone whose pronunciation strays a bit from personal norms, or those in the Liber…. or elsewhere.
Nevertheless, we should stick to the Roman style to the best of our ability.